1. Field of the Invention
The invention pertains to apparatus and methods for heat treating semiconductor wafers and more particularly to apparatus and methods for uniformly heating semiconductor wafers using microwave energy.
2. Description of Related Art
Single frequency microwaves have been explored for annealing ion implanted semiconductor wafers in the past. Heating semiconductors with microwave energy is very effective, leading to the interest of annealing wafers. However, as the size of the wafers has grown to 300 mm, used routinely today, uniform heating of the entire wafer is a challenge with fixed frequency microwaves. It will get even more difficult as the industry moves to 450 mm wafers. Furthermore, when it comes to placing metal components, circuits as well as metal coated wafers in a fixed frequency microwave cavity, the challenges are escalated: the arcing of metals tends to damage the circuits, and this becomes a major barrier to using this approach for the production of semiconductor devices.
One important heat treatment application involves annealing wafers to form metal silicides, which have been widely applied to IC fabrication because of their high melting points and low resistance. Use of fixed-frequency microwaves for this application has generally been unsuccessful. It should be noted that although single or fixed frequencies can in theory be used for microwave heating of semiconductors, they generally produce non-uniform heating, and when metal films are involved arcing with these films becomes a serious issue. However, pulsed microwave beam as described in U.S. Pat. No. 6,316,123, by Lee et al. has been used to locally heat and convert the metal to silicides. The pulse duration was of the order 0.02 to 0.15 seconds. This approach is similar to the laser spot annealing used on semiconductor wafers.
Metal silicides have been widely applied to IC fabrication. As the critical dimensions for contact area and source/drain regions become progressively smaller, nickel silicide is emerging to be the choice of material over cobalt and titanium silicide. However, the nickel silicide system has various phases and undergoes phase transformation during the heating cycle. Among all the phases, the lowest resistivity NiSi is the desired silicide phase for contacts to a semiconductor device. Thus there is the need to make sure that there is no temperature variation on the wafer so that the same phase is formed over the entire surface. Higher or lower temperature will alter the phase formation and hence the resistivity of the silicides.
Another important application involves the annealing or activation of dopant species in the silicon wafer following ion implantation, used for fabricating UltraShallow Junctions (USJ) and devices. The annealing process repairs the implantation damage and activation provides good conductivity. The key elements in forming USJ are junction depth and sheet resistance, and process manufacturability and repeatability. These shallow junctions demand low thermal budgets, requiring processing at a high ramp rate with a minimum of peak temperature overshoot. In a high-volume production environment it is critical to measure and control temperature for any thermal process. Lamp-based RTP spike-anneal has enabled recent production while laser spike-anneal (LSA) is emerging and even being claimed as the process of record for current high performance semiconductor device manufacturing. Generally, these are very short duration processes and there are challenges in measuring and controlling peak temperatures in spike-anneal process. The high temperature spikes may also lead to wafer warpage and strain in the device structure.